1. Field of the Invention
Aspects of the invention relate to a light sensing circuit, a touch panel including the same, and a method of driving the light sensing circuit, and more particularly, to a light sensing circuit that can accurately detect the brightness of light incident on a photodiode, a touch panel including the light sensing circuit, and a method of driving the light sensing circuit.
2. Description of the Related Art
In general, input devices, such as a mouse or a keyboard, are used to input a command to a computer. A remote control, which is also an input device, is used to control a device, such as a digital television, so that a user can select a specific function. However, users who are not skilled in using such a mouse, keyboard, or remote control have difficulty using these input devices.
A touch panel or touch screen is an input device that has been suggested as an attempt to solve the above problem. A touch panel is an input device that allows a user to input a command by directly touching a display panel with his or her finger or a pen instead of using a mouse, a keyboard, or a remote control.
Since a command can be input by touching a touch panel with a finger or the like, users having difficulty using input devices, such as a mouse, a keyboard, and a remote control, can easily use a digital device, such as a computer, by using such a touch panel to input commands. There are different types of touch panel technology that differ according to a method of recognizing an input: including capacitive technology, resistive overlay technology, infrared beam technology, surface acoustic wave technology using ultrasonic waves, strain gauge technology, piezoelectric technology, and light sensing technology.
Light sensing technology involves forming a photodiode in a display panel and sensing current generated due to light incident on the photodiode to recognize a touch by a finger. Such light sensing technology has advantages in that a touch panel can be simply manufactured because a photodiode used in the light sensing technology can be formed at the same time as a driving circuit of a display panel, such as a liquid crystal display (LCD) or an organic light emitting diode (OLED) display, is formed, and the thickness of the touch panel can be reduced since an additional layer does not need to be formed on the display panel unlike resistive overlay technology or capacitive technology.
However, the current generated by the photodiode is affected by the temperature of the photodiode or the temperature of ambient air surrounding the photodiode as well as by the brightness of the light incident on the photodiode.
FIG. 1 is a graph showing a relationship between a dark current flowing in a photodiode and a reverse voltage applied across the photodiode at different temperatures. The dark current is a current that flows in the photodiode when the reverse voltage is applied across the photodiode and no light is incident on the photodiode. In FIG. 1, the X-axis represents the reverse voltage applied across the photodiode in volts (V), and the Y-axis represents the dark current flowing in the photodiode in amps (A). The upper curve is obtained at a temperature of 40° C., and the lower curve is obtained at a temperature of 25° C.
Referring to FIG. 1, as the temperature increases, a larger amount of dark current flows at the same reverse voltage. Accordingly, as the ambient temperature changes, the amount of current flowing in a photodiode included in a touch panel changes because the current flowing in the photodiode when light is incident on the photodiode is a sum of the dark current and a current that depends on the brightness of the light incident on the photodiode. Hence, although light with the same brightness is incident on the photodiode, the detected brightness as indicated by the current flowing in the photodiode varies depending on the ambient temperature, which can cause errors in determining whether a user has touched the touch panel.